For portable communication devices such as two-way radios and pagers, the current trend in radio design is towards product miniaturization. One of the largest components in the radio, is the antenna. To reduce the antenna size, one solution is to use conventional micro-strip antennas, where the resonators are printed on a substrate using conventional thick or thin film processing.
Another trend in radio design is to use one broad-band antenna for multi-frequency operation. Since one antenna would eliminate the inconvenience of storing multiple parts, a low-profile broadband antenna is desired. However, micro-strip antennas (resonators) are inherently narrow band. To broaden a single microstrip antenna, one solution has been to stack a set of microstrip antennas of different resonant frequencies on top of each other. In this way, the resonant frequencies of each antenna combine to simulate a broadband frequency response.
Unfortunately, stacked antennas along with the associated matching network increase the thickness of the antenna. In many radios there is less room for a thickness increase than a width increase.
In addition, exciting multiple resonators requires multiple individual feeds. Often, the feed is accomplished by a feed probe that protrudes through a dielectric layer. For manufacturing simplicity, drilling through dielectric layer is not favored. Therefore, a low-profile broadband antenna with a single external feed is desired.